Hi Hels,
The zero beat point is where you should stand relative to the pitch antenna when playing.
When your hand is close to the antenna, it makes a high pitched noise. As you move your hand away the pitch gets lower and lower, until it is so low that you can't hear it any more. That is the zero beat point. As you move you hand further away the sound returns and you have left the zero beat point.
Think of it as being like the point where light comes to a focus when it passed through a lens.
This is how to adjust the theremin's settings so that the zero beat point is where you want it to be.
The idea of tuning the theremin is to get the zero beat point a comfortable distance away from the antenna so that when you stand with your hands by your side the theremin makes no noise, but just moving your right hand forwards a bit from your chest causes it to make a low growl (the theremin - not your chest!), rising to a high squeal when you almost touch the antenna, which should be a comfortable reach.
With the Kees, think of the knob at the back as the coarse tuning, and the knob at the front (number two if numbered from left to right) as the fine tuning.
Stand in the right position to play the theremin. Put the front knob so the white marker is at 12 o'clock, and then, keeping your right arm by your side, reach over the theremin with your other arm and turn the knob slowly until you get the pitch as low as possible. Now, also with your left hand turn the fine tuning knob until the noise stops completely.
Finally put your right hand up to your breastbone as if playing a very low note and just shuffle about a tiny bit to find exactly where the edge of the zero beat is.
If everything is working as it should you should now be ready to play...
Hope this helps.
Gordon